1. Field of the Invention
This invention relates generally to a transceiver provided in an earth station of a satellite communications system, and more specifically to such a transceiver suitable for use in a satellite communications system utilizing FDMA (frequency-division multiplexing access).
2. Description of the Prior Art
The FDMA is one of the multiple-access techniques currently used in satellite communications. A frequency-division multiplexing (FDM) is a transmission mode in which a plurality of signals are sent simultaneously using a different carrier frequency for each signal. These carrier frequencies are selected so that the signal spectra do not overlap. It is often the case, in an FDMA communications system, that each of the carrier frequencies received at an earth station deviates from the preset value by about 10 kHz to 100 kHz, for example. The principal cause of this frequency deviation is a frequency drift in a local oscillator which is provided in a frequency converter of a transponder on board a satellite. On the other hand, a frequency drift within an earth station is usually as small as 5 kHz.
Assuming that each satellite communication channel is separated from the adjacent one by 25 kHz and also assuming that the frequency deviation incurred through satellite transmission is between 10 kHz and 100 kHz, then there is a possibility that an earth station fails to receive the channel assigned thereto and erroneously receives either the one immediately adjacent thereto or the one which is separated by one or two channels.
In order to remove such a frequency deviation common to each channel, it is a known practice to utilize a pilot signal which is transmitted from a reference earth station. Each of the other earth stations receives the pilot signal via a satellite and the determines amount of a frequency deviation. Since the correct frequency of the pilot signal is known, each of the earth stations is able to compensate for the departure of each of the carrier frequencies.
However, this prior art technique has encountered the drawback that each of the earth stations has to be equipped with a receiver for exclusively receiving the pilot signal in addition to the usual receiver for normal communications. Thus, the prior earth station transceiver is bulky, complicated and expensive to manufacture.